The composition of raw natural gas varies but commonly contains 75 to 85% methane together with 5 to 10% of ethane and smaller amounts of higher hydrocarbons. Ethane and higher hydrocarbons can be effectively utilised for olefin production using current technology. However, there is no significant amount of ethylene produced commercially from methane at the present time. This is due to the high stability of the methane molecule compared to other alkanes making conversion of methane to ethylene extremely difficult.
Processes involving pyrolytic dehydrogenation of methane have been proposed. However severe reaction conditions, particularly temperature, are required. In addition the process is endothermic. The severity can be reduced by the use of catalysts, but methane conversion and selectivities are still low.
An alternative approach is to react methane with oxygen in the presence of a contacting material or catalyst to produce higher hydrocarbons.
The process is generally known as "oxidative coupling". The reaction pathway has been shown to involve the oxidative extraction of a hydrogen atom from a molecule of methane by the catalyst to form a methyl (CH.sub.3) radical followed by a coupling of two CH.sub.3 radicals to form the primary product ethane. The desired product ethylene arises from further reactions of ethane which may or may not involve surface reactions. Other higher hydrocarbons such as propane, propylene, butylene etc. are formed in smaller amounts. Undesired products such as carbon monoxide and carbon dioxide are also formed together with water and hydrogen. The source of the carbon oxides is still uncertain but they are thought to arise initially in parallel with ethane formation and also by secondary reactions of the product hydrocarbons with oxygen either on the catalyst surface or in the gas phase.
The contribution of the catalyst to the product spectrum is uncertain and unpredictable. Furthermore, as the mechanism by which the activation of the methane molecule by the catalyst surface is largely unknown, there is at present no known method for predicting catalyst performance.
European Patent Application No. 86103623.4, Publication No. 0196541 in the name of Phillips Petroleum Company discloses oxidative coupling catalysts comprising magnesium oxide and a compound of lithium. It is also known that compounds of other Group IIA elements do have some catalytic effect on the oxidative coupling of methane. However as described in the abovementioned European Patent Application magnesium oxide is not as effective as a catalyst consisting of a mixture of magnesium oxide and a compound of lithium.